1. Field of the Invention
This invention pertains to the field of pre-stressed pre-tensioned precast concrete slabs to be used for paving in areas subject to vehicular traffic.
2. Descrition of the Prior Art
Prestressed concrete is a mode of construction that overcomes concrete's inherent weakness in tension. When concrete is prestressed using one of three means available, longer spans can be created as measured against ordinary reinforced concrete. Traditional reinforced concrete uses steel rebar or other reinforcement material disposed within the concrete to reinforce it. Typically a swimming pool bottom is made in this manner. Prestressed concrete employs cables or strands to provide a clamping load which produces a compressive stress that can balance the tension stress that the concrete member would otherwise exhibit due to a bending load.
Pre-stressed concrete can be either pre-tensioned, or post tensioned. Pre-tensioned concrete is cast around already tensioned tendons. The concrete is poured around the pre-tensioned cables or tendons, and the concrete adheres to the tendons or cables as the concrete hardens during the curing process. When the tension is released from the tendons/cables this tension is transferred to the hardened concrete and compression by static friction thus creating concrete in compression. To achieve the pre-tensioning, anchor points are attached on opposite ends of the casting bed, between which, the tendons or wires are stretched in a straight line. When the tension is released, the tension is transferred to the hardened concrete unit by static friction.
Post-tensioned concrete is the mode for applying compression after the pouring and curing in situ of the concrete. There are two modes of doing so, one is called Bonded and the other is called Un-bonded.
In the bonded version, plastic, steel, or aluminum ducts, or tubes are laid out in a finite area, and the concrete is poured over and around the series of parallel tubes. Post tension cables are deployed through the tubes. Once the concrete hardens, the tendons are anchored at one end and tensioned at the other end using hydraulic jacks or rams that now react against the hardened concrete. After reviewing the design specification to confirm that adequate tension has been placed on the tendons, the jacks are removed such that the tension is now applied directly to the concrete member. The ducts or tubes are then grouted closed to protect the tendons from corrosion and decomposition. Concrete slabs prepared in this manner are usually used for bridges and house construction for slabs on grade in areas where the soil is expansive.
In the unbonded system each individual cable has freedom of movement relative to the concrete at all times. Each individual tendon is coated with a grease, often lithium based, and perhaps molybdenum sulfide would work also. Then the tendons are covered by an extruded plastic sheathing. The tension transfer arises from the tendons being connected to anchors embedded in the perimeter of the cast concrete slab. While the generalized discussion of post-tensioning serves as an introduction to the topic, more information can be obtained from the Post-tensioning institute which in the year 2011 is located at 8601 North Black Canyon Highway in Phoenix Arizona.
Pre-stressed, Pre-tensioned concrete can not only be used for buildings, but is used today in Bridge work and the manufacture of roads. Pre-stressed paving slabs can be laid into position during off-peak hours on nights and weekends. This minimizes lane closures, which can cause huge traffic backups, especially on highly traveled interstate freeways. The big advantage of using pre-stressed concrete slabs, is the relative speed of placement on site, less cracking, and the ability to use relatively thinner and longer slabs. Longer slabs reduce the number of joints that must be maintained. Basically whereas standard construction can take weeks for a project, the same project can be carried out in days using pre-stressed, pre-cast concrete slabs.
Numerous patents that relate to a method of forming, installing and a system for attaching prefabricated pavement slab to a subbase, and to the pavement slab itself have been issued to Peter J. Smith and said patents have been assigned to the Fort Miller Group, Inc. of Greenwich, N.Y. Some of these patents include:
U.S. Pat. No. 6,709,792Issued Mar. 24, 2004U.S. Pat. No. 6,607,329Issued Aug. 19, 2003U.S. Pat. No. 6,899,489Issued May 31, 2005U.S. Pat. No. 6,962,462Issued Nov. 8, 2005U.S. Pat. No. 7,004,674Issued Feb. 28, 2006 andU.S. Pat. No. 7,467,776Issued Dec. 23, 2008
Another inventor in this technology is Alfred A. Yee, whose two patents are assigned to Kwik Slab, LLC of Honolulu, Hi. His patents are U.S. Pat. No. 7,134,805 which issued on Nov. 14, 2006 and the published application 2005/0220539.
The Fort Miller Group product(s) are sold under the brand Super Slab, whereas the Yee products are sold under the brand Kwik Slab. It is believed that none of the aforementioned eight references singly or in combination disclose or render obvious the invention of this current patent application.
The reason that this assertion can be made is that the invention of this patent application relates to an entirely new technique for pre-stressing, pre-tensioning concrete slabs in 2 directions, not just one direction as has been the case with the prior art techniques.
As hinted above, the invention herein relates to a procedure for pre-stressing, pre-tensioning concrete slabs both longitudinally and transversely. The process further relates to the utilization of these bi-directionally pre-stressed, pre-tensioned slabs in the laying of roadways.
In order to better understand this invention it is necessary to lay the foundation—no pun intended—of the general technique for making roadway sections. As noted above, pre-stressing can be accomplished by pre-tensioning or post tensioning. Pre-tensioning is done in the concrete casting bed, prior to the pouring of concrete, while post tensioning is done after concrete is poured and sufficiently hardened. Most concrete roadways are normally laid in up to 224 foot lengths between expansion joints. These sections are made of a plurality of slabs 12 feet wide and 8 foot long. These slabs can be connected by a variant of a tongue and groove connection or some other type of joint. The joints are then grouted or otherwise treated to form a complete section of concrete roadway. This means that in this 224 foot span there will be 28 grout joints. 8 feet long×28=224 feet.
Generally pre-stressing in the concrete casting bed of a 12 foot length is carried out by pre-tensioning in the 12 foot direction prior to the pouring of the concrete and post-tensioning through the use of tendons or wires in a duct system after installation. But the pre-tensioning in the prior art techniques is in only one direction, longitudinally. The process of this invention is pre-stressing, pre-tensioning the concrete in both directions, longitudinally and laterally using a pre-tension technique longitudinally and laterally. Optional post-tensioning may also be applied. This allows for the preparation of longer slabs, potentially up to 60 feet in length, thereby minimizing the number of joints to be grouted and maintained in each roadway section, and thus speeding up the installation process.
The invention accordingly comprises the apparatus (casting bed) and the device (dual direction, pre-stressed, pre-tensioned) concrete slab and the process of making the device, each of which possesses the features, properties, the selection of components which are amplified in the following detailed disclosure, and the scope of the application of which will be indicated in the appended claims.